Cathode ray tube (CRT) displays, such as those commonly used in desk-top computer screens, function as a result of a scanning electron beam from an electron gun impinging on phosphors on a relatively distant screen. The electrons increase the energy level of the phosphors. When the phosphors return to their normal energy level, they release the energy from the electrons as a photon of light which is transmitted through the glass screen of the display to the viewer. One disadvantage of a CRT is the depth of the display required to accommodate the raster scanner.
Flat panel displays have become increasingly important in appliances requiring lightweight portable screens. Currently, such screens use electroluminescent or liquid crystal technology. Another promising technology is the use of a matrix-addressable array of cold cathode emission devices to excite phosphor on a screen, often referred to as a field emission display. To produce the desired field emission, a potential source is provided with its positive terminal connected to the gate, or grid, and its negative terminal connected to the emission electrode (cathode conductor substrate). The potential source is variable for the purpose of controlling the electron emission current. Upon application of a potential between the electrodes, an electric field is established between the emission tips and the low potential anode grid, thus causing electrons to be emitted from the cathode tips through the holes in the grid electrode.
The clarity, or resolution, of a field emission display is a function of a number of factors, including emission tip sharpness, alignment and spacing of the gates, or grid openings, which surround the tips, pixel size, as well as cathode-to-gate and cathode-to-screen voltages. These factors are also interrelated. For example, the voltage required for electron emission from the emission tips is a function of both cathode-to-gate spacing and tip sharpness. A relatively sharper emission tip may both improve resolution and lower power consumption.
Existing techniques for sharpening the emission tip typically involve an oxidation process followed by an etch process. The surface of the semiconductor substrate, such as silicon, and the emission tip are first oxidized to produce an oxide layer of SiO.sub.2, which is then etched to sharpen the tip. The oxidation process is ordinarily either a wet or a dry process. In a dry oxidation process, the substrate and emission tip are exposed to an atmosphere containing a significant percentage of gaseous oxygen at temperatures of 800.degree. C. or more. In a wet oxidation process the substrate and tip are exposed to steam at around 800.degree. C.
In either existing oxidation technique, there is the risk that the oxidation process itself will induce flow of silicon and oxide, that is, cause the silicon and the forming oxide layer near the top of the emission tip to, in essence, flow down the sloping sides of the tip. This flowing action results in an undesirable rounding of the tip. In a dry process, oxidation typically does not appreciably occur below 800.degree. C. However, at temperatures above 800.degree. C., flow of silicon and oxide can readily occur. A wet process will usually grow a sufficient oxide layer at 800.degree. C., however, the chemical nature of existing wet processes can nevertheless lead to significant flow of silicon and oxide.